In recent years, optical disks, such as a CD (Compact Disk) and a DVD (Digital Versatile Disk), have become prominent as an information recording medium. A reproducing device for these optical disks emits a laser beam along a track of the optical disk by means of an optical pickup mechanism and detects the reflected light. Recorded data is then reproduced in accordance with a change of the reflected light intensity.
The optical disk reproducing device, while detecting the data based on the reflected light, servo-controls a positional relationship between the optical pickup mechanism and the optical disk. Specifically, it performs a tracking servo control for emitting the laser beam along with a centerline of the track and a focus servo control for keeping a distance between the optical disk and the optical pickup mechanism constant. For example, the focus servo control variably controls a position of the optical pickup mechanism with an actuator, based on an output signal from a photodetector for detecting the reflected laser beam, to thus keep a distance d between the optical pickup mechanism and the optical disk constant. As a result, a fluctuation in the amount of the reflected light in accordance with a focus offset of the emitted light on the surface of the optical disk is suppressed, and thus noise superimposed on a light receiving signal is suppressed.
In order to acquire information for such servo control, there is used a device as a photodetector for dividing a reflected light image into a plurality of segments to thereby receive them. FIGS. 1 to 3 are schematic diagrams illustrating light receiving sections of the photodetectors and the reflected light images on the light receiving sections. The reflected laser beam enters into the photodetector through a cylindrical lens. The reflected light has a circular cross section when entering into the cylindrical lens. Based on a principle of an astigmatism method, after passing through the cylindrical lens, a dimensional ratio of two directions orthogonal to each other in the reflected light image is changed, in accordance with the distance d between the optical pickup mechanism and the optical disk. Specifically, if the distance d is a target value, the reflected light image is set to be a perfect circle 10 as shown in FIG. 2. Meanwhile, if the distance d is above the target value for example, the reflected light image will be a vertically elongated ellipse 12 as shown in FIG. 1, while the reflected light image will be a horizontally elongated ellipse 14 as shown in FIG. 3 if the distance d is below the target value.
The photodetector has the light receiving section divided into four (2×2) segments 16, wherein each segment constitutes a light receiving element which produces the light receiving signal. The photodetector is arranged so that diagonal directions of a 2×2 square array of the light receiving element are coincident with axes of the vertically elongated ellipse 12 and the horizontally elongated ellipse 14, respectively. Arrangement in such a way allows for determination of a shape of each reflected light image based on a difference between the sum of the output signals from the two light receiving elements aligned along a diagonal line in the vertical direction and the sum of the output signals from the other two light receiving elements aligned along the diagonal line in the horizontal direction, in FIGS. 1 to 3. The shapes of the reflected light images can be used for controlling the distance d. The reflected light intensity in accordance with the data will be calculated from the total of the output signals from the four light receiving elements.
Since a data rate read from the optical disk is extremely high, the photodetector is constituted of a semiconductor device using a PIN photodiode with high response speed. A small photoelectric conversion signal generated in the light receiving section is amplified with an amplifier and outputted to a subsequent signal processing circuit. Here, an interconnection length between the light receiving section and the amplifier is made as short as possible from the viewpoint of preventing attenuation of the photoelectric conversion signal and superimposition of the noise. From the viewpoint above and the viewpoint of reduction in manufacturing cost of the photodetector, the light receiving section having a PIN photodiode structure, and the circuit section including the amplifier or the like are preferably formed on the same semiconductor chip.
FIG. 4 is a schematic sectional view of the photodetector with the light receiving section and the circuit section which are disposed adjacent to each other on the common semiconductor substrate. The PIN photodiode structure is formed in a region corresponding to a light receiving section 22 of a semiconductor substrate 20, while circuit elements such as a transistor or the like are formed in a region corresponding to a circuit section 24 thereof. An anode region and an i layer with high resistivity of the PIN photodiode are constituted of a p-type region (P-sub 26) of the semiconductor substrate 20 and an epitaxial layer 28 on a surface side of the semiconductor substrate 20, respectively. A cathode region of the PIN photodiode is constituted of an n+ region 30 which is formed by diffusing impurities on the surface of the i layer.
The photodetector shown in FIG. 4 has a structure in which two interconnection layers are formed on the semiconductor substrate 20. A first interlayer insulating film 40 is deposited on the surface of the semiconductor substrate 20, and an aluminum layer (a first Al layer) serving as a metal film is then deposited thereon. The first Al layer is patterned using a photolithography technique, and a first interconnection layer 42 and a planarizing pad 44 are formed in the circuit section 24 using this first Al layer. Here, the planarizing pad 44 is disposed in a region between the interconnections 42 to reduce irregularity on a surface of an interlayer insulating film 46 which is to be subsequently laminated thereon. A second Al layer is then deposited on the surface of the interlayer insulating film 46, and by patterning it, a second interconnection layer 48 and a planarizing pad 50 are formed in the circuit section 24. The planarizing pad 50 serves similarly to the planarizing pad 44. In other words, it is disposed in the region between the interconnections 48 to reduce irregularity on a surface of an interlayer insulating film 52 which is to be subsequently laminated thereon. As a result, by planarization of the interlayer insulating films using the planarizing pads, the heights of the interlayer insulating films 46 and 52 can be equalized over the circuit section 24. On the interlayer insulating film 52 in the circuit section 24, an Al layer 54 for light shielding is deposited, and a silicon oxide film 56 and a silicon nitride film 58, both serving as a protective film, are deposited sequentially.
Thereafter, in order to improve a light incidence efficiency to the semiconductor substrate in the light receiving section 22, an etch back process is performed to reduce a thickness of the layer stack on the semiconductor substrate in the light receiving section 22. In FIG. 4, dotted lines represent portions to be etched off among the respective layers stacked on the light receiving section 22. Namely, a silicon nitride film 58′, a silicon oxide film 56′, an interlayer insulating film 52′, and an interlayer insulating film 46′ are etched off, so that a protective layer 60 on the surface of the light receiving section 22 is formed of the remaining interlayer insulating film.
The interlayer insulating films 46 and 52 are formed by applying a material having fluidity, such as a SOG (Spin on Glass), using a spin-coat method or the like. Here, since the metal films which constitute the interconnection and the planarizing pads are disposed in the circuit section 24, a step is caused at the boundary between the light receiving section 22 and the circuit section 24 according to the presence/absence of the metal film. The applied material tends to be accumulated in a recess (internal corner) of the step. Consequently, the interlayer insulating film at a periphery of the light receiving section 22 will be thicker than that at a central portion of the light receiving section 22 before performing the etching process. Particularly, in the constitution described above where the metal films are left with relatively high area density in the circuit section 24 in order to reduce the irregularity of the interlayer insulating films, the surface of the interlayer insulating films remains high in the circuit section 24 because of the thickness of the metal films. Due to this, the thicknesses of the interlayer insulating films 46 and 52 increase at the periphery of the light receiving section 22, and an area where the interlayer insulating films are thick extends inward in the light receiving section 22. A profile of the surface of the interlayer insulating film 52 formed as described above is reflected to a profile of the protective film to be deposited thereon, and is further reflected to a profile of the protective layer 60 in the light receiving section 22 after the etching process. Consequently, an area at the periphery of the light receiving section 22 where the protective layer 60 is thick extends relatively inward in the light receiving section 22, similarly to the protective layers 46 and 52. Therefore, there has been a problem that the influence of attenuation of the incident light at the periphery of the light receiving section 22 and the influence of refraction of the incident light due to the protective layer surface being inclined at the periphery have increased, and thus the nonuniformity in photo sensitivity within a light receiving surface could be significant. Particularly, the more the amount of etch-off increases, the more a ratio between the thicknesses at the central portion and at the periphery of the protective layer 60 increases, so that there is a difference in attenuation between the incident light at the central portion and at the periphery of the light receiving section 22.